POWER GENERATION AND TRANSMISSION 77 pipe is generally used, with special hydraulic valves and fittings. For screwed-end work, fittings may be forged-steel blocks. For large flanged work, cast-steel fittings with tongued and grooved flanges give good service. Storage of fluid under pressure may be provided by an accumulator or sheet-iron. tank filled with a heavy material like hammer-scale, and attached to the top of a plunger which floats in a cylinder. If d = diameter of plunger, in., h = rise of accum- ulator, in., the volume of fluid stored is -^d^h and the weight of the accumulator is -rpwd2 Ib. Pump regulation is effected by trips which are thrown when the accum- ulator rises or falls. The losses through the accumulator should not exceed 5 per cent. 13 3000 5500 3500 4000 4500 5000 Speed of Ropes,ft.per mm. Fia. 37.óRope-drive constants. In all cases, the pressure used should be the lowest which will do the work. This saves first cost, power and time. In many instances a part of an operation can be performed at low pressure, the finish only requiring high pressure. This dual system may be provided by a complete duplication of pumps, accumulators and piping, with manual or automatic transfer valves, or intensifiers may be used at points of fluid consumption. These are pumps using the low-pressure fluid as the actuating medium in what corresponds to the "steam" cylinder, to produce a local excess pressure. Where part of the equipment requires large drafts of fluid (as for instance the flanging presses in a boiler shop) it is permissible to use a dual system with only a slight difference in pressures. The higher pressure fluid goes to supply the steady load and the surplus is vented to the low-pressure system.